# Math Help - expected value and variance

1. ## expected value and variance

okay I am having some issues (i think) withthe expected value and variance of my PDF.

Let me see if I have it right.

My pdf is 2/(x^3) from 1 to 2000.

I get my mean to be 2 and my
variance to be 2 log(2000)-4. Is that correct? If you look at the graph it doesnt seem to be right (the mean is the right solid red bar.) maybe it is because it just extends so far?

2. Originally Posted by xamius
okay I am having some issues (i think) withthe expected value and variance of my PDF.

Let me see if I have it right.

My pdf is 2/(x^3) from 1 to 2000.

I get my mean to be 2 and my
variance to be 2 log(2000)-4. Is that correct? If you look at the graph it doesnt seem to be right (the mean is the right solid red bar.) maybe it is because it just extends so far?
The given pdf is not valid: it does not integrate to 1.

3. Originally Posted by mr fantastic
The given pdf is not valid: it does not integrate to 1.
I get one whether by hand or on wolfram alpha...

4. Originally Posted by xamius
I get one whether by hand or on wolfram alpha...
Really? Strange then that I don't.

My 'by hand' calculation is in agreement with integrate 2&#47;x&#94;3 from 1 to 2000 - Wolfram|Alpha

Approximately 1 is NOT the same as 1. It is NOT a valid pdf. There is no point in doing the question.

5. okay I see what you mean. I now see the PDF is (8000000/3999999)*(1/x^3)

6. Originally Posted by xamius
okay I see what you mean. I now see the PDF is (8000000/3999999)*(1/x^3)
So the pdf was not the question? Your pdf was your answer to some other question? It would help if the original question was posted.

7. well after that correction things turned out looking better for the mean and such. thanks for the help